Handling emergency service calls made by subscribers pose several hurdles not encountered with, for example, land line devices. FIG. 1 illustrates an example of one of these hurdles. Consider the situation in which an emergency service call originates from a wireless unit WU while moving in a high-speed train. If the call is somehow disconnected or dropped before the emergency is completely reported, the Public Service Answering Point (PSAP) to which the emergency call was routed will attempt to call back the originator of the emergency call. However, as shown in FIG. 1, the caller may have moved from the coverage area 10 of an original wireless serving system to the coverage area 12 of a border system sometime after the call was dropped but before the call back could be completed. As a result, the wireless unit might not be reachable by the border system to deliver the call back. A border system is one of a group of other wireless systems, the identity of which is determined by, for example, a network operator. Border systems do not necessarily have coverage areas bordering the original serving system.
Current solutions for emergency call back to a wireless unit that has moved into a border system only work for when a mobile directory number (MDN) associated with the wireless unit is known at the PSAP. The MDN of a wireless unit is a dialable number. The MDN is dialed by a caller and used to route a call through the network to the wireless subscriber's home system. At the subscriber's home system, the home location register (HLR) contains the mobile subscriber identifier MSID associated with the subscriber's MDN. The MSID, not the MDN, is then used to route the call through the network to the serving wireless system and page the subscriber. The subscriber's MDN is provided by the home system to the serving system in a separate data file called the subscriber profile. Typically, the MSID is either a 10-digit mobile identification number (MIN) or a 15-digit International Mobile Subscriber Identifier (IMSI) programmed into a wireless unit by the service provider with whom the wireless unit user has entered into a service agreement. Accordingly, the MSID is not necessarily a dialable number.
In one solution for emergency call back where a wireless unit has moved into a border system, the emergency call back is routed to the wireless unit through the wireless unit's home system, which then delivers the call to the border system according to existing standards. This process involves the use of temporary local directory numbers (TLDNs) signaled from the border system to the home system; wherein the TLDN assigned to the emergency call back is used by the home system to deliver the call to the border system.
As will be appreciated, this can be a time consuming process, particularly for an emergency call, and is especially acute if the wireless unit is an international roamer or the home system and border system are separated by great distances.
Furthermore, as alluded to above, there is no solution for when the MDN of the emergency caller's wireless unit is unknown. The MDN could be unknown for many reasons, including (a) the wireless unit was never intended to be registered (there are such phones to use for emergency calls only), (b) the phone is new and has not yet been initialized by a service provider or (c) the subscription has expired and the wireless unit is no longer registered with a service provider. Some mobile phones also support a removable User Identity Module (R-UIM) or Subscriber Identity Module (SIM) that may contain the MSID and the MDN. If the R-UIM or SIM are not in the wireless unit, then the wireless unit can still be used to place an emergency call. However, there is no MDN or MSID known to the wireless unit or the serving system to provide the PSAP as a call back number.